Hitherto, in an image forming apparatus such as a multi-function color copier (MFP), the types of media on which color images or the like are printed are increased. Even if the print media are limited to sheets, various sheets different in thickness or the like are used.
Incidentally, the types of sheets are generally differentiated by basis weight (unit: g/m2) indicating the weight per an area, and for example, sheets of groups of 64 to 105, 106 to 163, 164 to 209, 210 to 256, and 257 to 300 are respectively called standard paper, thick paper 1, thick paper 2, thick paper 3 and thick paper 4. These basis weights are generally written on packages of sheets. When the user selects one of the groups according to the written basis weight, a print condition corresponding to the type of the paper is automatically set. In general, it is desirable to change the print condition according to the basis weight. However, there is a case where the basis weight is unclear, and in such a case, it is difficult to simply detect the basis weight. The basis weight can be calculated from the density of the sheet and the paper thickness. Although it is difficult to measure the density of the sheet, it is relatively easy to measure the paper thickness as compared with the measurement of the density. Then, an apparatus is known which detects the paper thickness in order to find the paper type or the like.
There are various such paper thickness detecting apparatuses. JP-A-2003-237982 discloses a paper thickness detection apparatus in which a sheet is made to pass through between a rotating drive roller and a paper thickness roller, and the paper thickness is detected by using that the paper thickness roller shifts around an arm shaft at that time.
Besides, JP-A-2003-269904 discloses a paper thickness measuring apparatus in which a sheet is inserted and made to pass through between a fixed section (ferrite) made of a magnetic material and a rotation section, and the paper thickness is measured by using the amount of magnetic field change at the time of passing. The paper thickness measuring apparatus is provided at a halfway portion of a sheet conveyance path, and is disposed immediately after (downstream side in the paper conveyance direction) a nip of a paper conveyance unit including a conveyance roller 9a and a conveyance roller 9b (see paragraph [0016]).
After the paper thickness is measured by such a paper measuring apparatus, the sheet is conveyed to a register roller and the sheet is aligned.
When recording is performed continuously, in order to increase the number of copied sheets per unit time, it is desirable that the sheets are fed from the paper thickness detecting apparatus to the register roller without interruption and as continuously as possible.
The sheet is aligned by the register roller, and the skew of the sheet is removed. However, when the trailing edge and its vicinity of the sheet remains in the paper thickness detecting apparatus when the sheet is aligned, this portion slides in the lateral direction relative to the running direction of the sheet. At this time, when the paper thickness detecting apparatus is such an apparatus that the paper thickness is detected by the shift of an outer ring of a bearing section, friction occurs between the sheet and the outer ring by the sliding of the sheet in the lateral direction (thrust direction), and an abnormal sound may be generated.